A major area of interest in genome biology, especially in light of the determination of the complete nucleotide sequences of a number of genomes, is the targeted alteration of genome sequences. To provide but one example, sickle cell anemia is caused by mutation of a single nucleotide pair in the human β-globin gene. Thus, the ability to convert the endogenous genomic copy of this mutant nucleotide pair to the wild-type sequence in a stable fashion and produce normal β-globin would provide a cure for sickle cell anemia, as would introduction of a functional β-globin gene into a genome containing a mutant β-globin gene.
Attempts have been made to alter genomic sequences in cultured cells by taking advantage of the natural phenomenon of homologous recombination. See, for example, Capecchi (1989) Science 244:1288-1292; U.S. Pat. Nos. 6,528,313 and 6,528,314. If a polynucleotide has sufficient homology to the genomic region containing the sequence to be altered, it is possible for part or all of the sequence of the polynucleotide to replace the genomic sequence by homologous recombination. However, the frequency of homologous recombination under these circumstances is extremely low. Moreover, the frequency of insertion of the exogenous polynucleotide at genomic locations that lack sequence homology exceeds the frequency of homologous recombination by several orders of magnitude.
The introduction of a double-stranded break into genomic DNA, in the region of the genome bearing homology to an exogenous polynucleotide, has been shown to stimulate homologous recombination at this site by several thousand-fold in cultured cells. Rouet et al. (1994) Mol. Cell. Biol. 14:8096-8106; Choulika et al. (1995) Mol. Cell. Biol. 15:1968-1973; Donoho et al. (1998) Mol. Cell. Biol. 18:4070-4078. See also Johnson et al. (2001) Biochem. Soc. Trans. 29:196-201; and Yanez et al. (1998) Gene Therapy 5:149-159. In these methods, DNA cleavage in the desired genomic region was accomplished by inserting a recognition site for a meganuclease (i.e., an endonuclease whose recognition sequence is so large that it does not occur, or occurs only rarely, in the genome of interest) into the desired genomic region.
However, meganuclease cleavage-stimulated homologous recombination relies on either the fortuitous presence of, or the directed insertion of, a suitable meganuclease recognition site in the vicinity of the genomic region to be altered. Since meganuclease recognition sites are rare (or nonexistent) in a typical mammalian genome, and insertion of a suitable meganuclease recognition site is plagued with the same difficulties as associated with other genomic alterations, these methods are not broadly applicable.
Thus, there remain needs for compositions and methods for targeted alteration of sequences in any genome and for compositions and methods for targeted introduction of exogenous sequences into a genome.